Extent of Disorder in Magnesium Aluminosilicate Glasses: Insights from ²⁷Al and ¹⁷O NMR

Abstract

The quantification of the intrinsic disorder in archetypal noncrystalline magnesium aluminosilicates remains unsolved. This lack of knowledge is because of the increased structural perturbation caused by Mg 2+, a high field strength cation, resulting in substantial broadening in both spectral and scattering responses. Most progress regarding amorphous aluminosilicate has thus been made with relatively low field strength cations (e.g., Na + and Ca 2+ ). Here, we quantified the nature of structural disorder in Mg-aluminosilicate glasses in the enstatite (MgSiO 3 )-pyrope (Mg 3 Al 2 Si 3 O 12 ) join using 17 O and 27 Al NMR. While Mg-aluminosilicate glasses show a much larger topological and configurational disorder around Al than those of Na- and Ca-analogues, the fraction of [5,6] Al (∼8–10%) and the magnitude of topological disorder do not vary significantly with composition. This implies spatial proximity between Mg 2+ and the under-bonded bridging oxygens, such as Al-O-Al and Si-O-Al, while Mg 2+ preferentially forms Mg-O-Si over Mg-O-Al. The estimated degree of Al avoidance ( Q ) of ∼0.65 for Mg-aluminosilicates based on 17 O NMR is close to a random distribution of Si/Al ( Q = 0) and is thus much smaller than those estimated for Na- and Ca-aluminosilicate glasses (from ∼0.95 to ∼0.85) that often show evidence for Si/Al ordering ( Q = 1, complete Al avoidance). The results also revealed that degree of Al avoidance decreases linearly with increasing cation field strength of non-network-forming cations, highlighting the first simple predictive relationship between the nature of chemical disorder and the types of non-network forming cation. This established correlation can be utilized to explain and predict the diverse properties of the Mg-bearing multicomponent glasses and melts with complex composition-dependence.